Cryptography

Concept · Chapter 11

What It Is

The study and practice of keeping information secret.
Supports confidentiality, integrity, and non-repudiation, enabling secure communication. Two core tools: encryption and cryptographic hash functions.

Transforms plaintext into unreadable ciphertext using an algorithm + key; larger keys mean stronger but slower encryption.
Reversible: decryption recovers the original value. Use encryption (not hashing) when the decrypted value must be recovered — e.g. sending a secure message the recipient must read.
The algorithm pair is called a cipher. Two types:

Type	Also called	Keys	Notes
Symmetric	Secret key	One shared key for both operations	Faster; drawback: both parties must hold the secret key
Asymmetric	Public key	Public key (shareable) + private key (secret)	Either key encrypts, the opposite decrypts; slower

A hash function maps any-size input to a fixed-size output (the hash / message digest / digest). e.g. SHA-256 always yields a 256-bit (32-byte) value.
Not reversible — unlike encryption. Prefer hashing over encryption when the original value never needs recovering.
General uses: file-equality comparison, checksums, finding similar records, hash tables, Bloom filters.
A cryptographic hash function guarantees properties that make it secure (used for digital signatures, HTTPS certificates, SSL/TLS, SSH). Non-cryptographic hashes are faster but weaker.

Quick — fast to compute, or dependent processes degrade.
Deterministic — same input always yields the same hash (lets you compare hashes without knowing the input).
One-way — infeasible (impractical, not literally impossible) to recover a message from its hash without brute force.
Collision resistant — infeasible to find two inputs with the same hash. MD5 and SHA-1 have known collisions and must not be used for cryptography.
Avalanche effect — a tiny input change produces a wildly different, uncorrelated hash.